You would choose to advertise on the Google Display Network if you wanted to:
show ads to people on non-Google search sites
show ads on Google Maps
show ads on websites related to your business
show ads on Google Shopping
The Google Display Network allows to you connect with customers with a variety of ad formats across the digital universe. This network spans over two million websites that reach over 90% of people on the Internet. It can help you reach people while they’re browsing their favorite websites, showing a friend a YouTube video, checking their Gmail account, or using mobile sites and apps.
“Display Network only” campaigns are best if you’d like to show ads on websites and apps when your keywords are related to the sites’ content. Your ads are matched to websites or mobile apps (called placements) that include content that’s related to your business or to your customers’ interests. You can set up targeting to match your ads to the most relevant Display Network sites based on their topics, interested audiences, demographics, and more.
Read more here: https://support.google.com/adwords/answer/6340468?hl=en